In Visual Studio 2010, I have a XAML screen with a textbox defined like this:

                        < TextBox  Name="txtBalanceOutstanding"
                                  Grid.Row="9" Grid.Column="1" Grid.ColumnSpan="1" Height="23" 
                                  HorizontalAlignment="Stretch" Margin="0,0,100,0" 
                                  IsReadOnly="True" FontWeight="Bold" Focusable="False" VerticalContentAlignment="Center" >

                        < /TextBox >

When I change it to this (adding in the MultiBinding):

                        <TextBox  Name="txtBalanceOutstanding"
                                  Grid.Row="9" Grid.Column="1" Grid.ColumnSpan="1" Height="23" 
                                  HorizontalAlignment="Stretch" Margin="0,0,100,0" 
                                  IsReadOnly="True" FontWeight="Bold" Focusable="False" VerticalContentAlignment="Center">
                            <MultiBinding Converter="{StaticResource OutstandingBalanceConverter}">
                                <Binding ElementName="txtDepositTotal" Path="Text"></Binding>
                                <Binding ElementName="txtDep_Amount" Path="Text"></Binding>
                            </MultiBinding>
                        </TextBox>

The screen designer shows an unhandled exception error has occurred:

Unable to cast object of type 'MS.Internal.NamedObject' to type 'System.String'.
at System.Linq.Enumerable.< CastIterator >d__b1`1.MoveNext()
at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at SamaanSystems.IBC.WPF.OutstandingBalanceConverter.Convert(Object[] values, Type targetType, Object parameter, CultureInfo culture) in D:\Development\VS2010\IBC\IBC WPF\Converters\OutstandingBalanceConverter.cs:line 13
at System.Windows.Data.MultiBindingExpression.TransferValue()
at System.Windows.Data.MultiBindingExpression.Transfer()
at System.Windows.Data.MultiBindingExpression.UpdateTarget(Boolean includeInnerBindings)
at System.Windows.Data.MultiBindingExpression.AttachToContext(Boolean lastChance)
at System.Windows.Data.MultiBindingExpression.MS.Internal.Data.IDataBindEngineClient.AttachToContext(Boolean lastChance)
at MS.Internal.Data.DataBindEngine.Task.Run(Boolean lastChance)
at MS.Internal.Data.DataBindEngine.Run(Object arg)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

Any idea what I'm doing wrong?